Altman, D. G. Statistics in medical journals: developments in Altman, D. G. Statistical reviewing for medical journals. Sta t. Bailar, J. C., III. The real threats to the integrity of science. Bland, M. An Introduction to Medica l Sta tistics 2nd ed.. Bra

Source df Error term Group 1 Sgroup Test 1 Test 3 Sgroup Sgroup 38 Group 3 test 1 Test 3 Sgroup Test 3 Sgroup 38 Total 79 The statistical test selected was appropriate, but the biologist’s conclusions were wrong. If the researcher’s conclusions were correct, a significant group 3 test interaction would have been obtained. Subsequent analyses of this interaction should also have indicated that for the knockout mice the difference in errors made on the complex and simple tasks was greater than the trial error difference between these two behavioral tests for the normal mice. However, the group 3 test interaction, F1, 38 5 1.20, was not significant; consequently, the only conclusions the biologist could make would be interpretations based on the two reported main effects, namely, that the knockout mice required significantly more trials to learn the two mazes compared with their control litter- mates and that mice in both groups required significantly more trials to learn the complex than the simple maze. Therefore, no evidence exists that a deficiency in hippocampal function affected the mouse’s performance more on complex than on simple tasks. Issues that may be reviewed in addition to interaction interpreta- tion: •Mixed factorial design •Main effects and interactions •Between-subject and within-subject components •Confounds, carryover effects •Counterbalancing •ANOVA table, levels of a factor, definitions of a factor, df, and P value •Expected value of F-ratio •Assumptions of statistical test I am grateful to Brian Jackson, Daniel Richardson, and Eric Smart for helpful comments on this paper. I thank Kim Ng of Monash University for supporting the use of research design problems and their evaluations as a teaching method with a large class of advanced undergraduate students. Address for reprint requests and other correspondence: J. F. Zolman, Dept. of Physiology, MS 508C Medical Center, College of Medicine, Univ. of Kentucky, Lexington, KY 40536-0298. E-mail: jzolmanpop.uky.edu. Received 18 November 1998; accepted in final form 25 August 1999. Refer ences

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